Interconnecting signals with only one (hot) lead connected

Discussion in 'General Electronics Chat' started by CurlsOnKeys, Mar 2, 2016.

  1. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
    47
    2
    Ok, I have a simple question...

    I have an audio interface with a 3 prong plug and a speaker/amp with a 3 prong plug. Both have balanced connections. When I put a one-wire cable between them, only connecting the "hot" leads, everything, to my surprise, worked flawlessly. No noise, no mains hum, no nothing. Connecting the sleeve with a second cable made no difference. I was testing this because I wanted to demonstrate to my students that "current always has to return, so that's why cables have 2 leads: hot and return". Turns out I don't need the return... Luckily I tried it at home first :)
    My guess is that because it's balanced equipment with 3-prong plugs, the audio finds it return-path through the chassis/ground and since my whole studio is grounded from the same place, there can't be too much potential difference between two grounds (none as it seems), so the current return path here is actually earth ground, correct?

    When I did the same test with a synth that only has a 2 prong power plug (so no connection to mains ground) and plugged the one-wire cable into the speaker, all I got was mains hum. So I thought: aha, mains hum interference got picked up by the synth (= one large copper plate), couldn't escape to ground (no connection to mains ground on the 2 prong power) and found its way out of the synth towards the ground connection of the speaker, correct?

    But why am I, aside from the mains hum, not ALSO hearing the audio? I mean, the current/voltage that sits on my audio output hot lead could also flow towards the speaker and find its way to ground there, no? If mains hum (Europe, 50 Hz) can do it, why can't my audio signal (low note of 100 Hz) do the same thing? This is important for me to thoroughly understand, as I can demonstrate here why you DO need a current return path (cause without current return/second wire all you get is hum...). I'm guessing because ground/reference in my synth is different from ground/reference in the speaker and signal current has no intention to flow between them, does that seem likely?

    Thanks.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Sounds like you are using the chassis ground/earth ground instead of the neutral.. They are typically connected at one point (and should only be one point)
    What you have is a code violation and should not be done for safety reasons as current would be flowing through the chassis..
     
  3. CurlsOnKeys

    Thread Starter Member

    Jan 3, 2015
    47
    2
    I'm well aware it shouldn't be done, don't worry. I'm trying to understand what happens in a "test" situation (= carefully connecting, not touching chassis etc...), I'm not intending to actually do this in a "real" situation
    I know current is finding it's return path through the chassis when connecting two devices that are grounded. However, I was wondering what happens when connecting one device that's not grounded (a synth) to a device that is grounded (a speaker) with only the hot lead: what I hear is AC mains hum interfering on the speaker = AC interference current that's seeking its way out of the synth towards ground via the speaker. But why am I not seeing or hearing the actual synth sound, which should also find its way out of the synth towards that same ground? Again: by no means I'm intending to "use" such a setup, I'm measuring, visualizing and testing some things to try and better understand what really happens.

    Thanks!
     
  4. HitEmTrue

    Member

    Jan 25, 2016
    32
    9
    What exactly is the code violation? The one wire audio cable? Or is there something else in his setup that is not correct?
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    I "fast" read the OP's post initially but a code violation would be using a "safety" ground connection to carry normal operating current.
    Thats what I was referring to..
     
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